The present invention generally relates to aircraft electrical power distribution systems (EPDS) and more particularly to such systems which employ solid state power controllers (SSPC) for power routing and protection against damage from electrical fault conditions.
SSPC technology is gaining acceptance as a modern alternative to the traditional electromechanical contactors and circuit breakers, due to its high reliability, fast response time, and ability to facilitate advanced load management and other aircraft functions. A typical SSPC comprises a solid state switching device (SSSD) which performs fundamental power on/off switching and a SSPC processing engine which is responsible for SSSD on/off control and feeder wire protection. While SSPCs with current rating less than 20 A have been widely used in aircraft secondary distribution systems, replacing thermal circuit breakers, their application for aircraft high voltage DC (HVDC) power distribution systems has issues and challenges.
Unlike direct metal contacts in a traditional electromechanical contactor or relay, an SSSD for higher voltage and current applications has more significant voltage drop in the “on” state. This voltage drop leads to undesirable power dissipation across the SSSD, which presents significant challenges in the thermal design of SSSD packaging and may adversely impact the life, size and weight of an entire power distribution system.
An aircraft electric power distribution system (EPDS) is often required not only to survive, without any physical damage, when lightning strikes the aircraft, but also to maintain operation during and after the strike. This poses a significant challenge to the design of SSPC based EPDS, since SSPCs often contain electronic circuitry which could be damaged or upset by the excessive transient voltages induced resulting in undesirable (or nuisance) trips due to lightning strikes. Increasing use of composite materials in aircraft fuselage exacerbates the situation.
When a DC SSPC is turned on to a large capacitive load, the peak inrush current can be very high. This is particularly true for SSPCs used in a high voltage DC primary power distribution system, where large energy storage components are connected to the DC power bus, and when multiple downstream SSPC channels are turned on simultaneously during power-up process. Excessive high inrush current could result in stress in electrical components during power-up, therefore reducing their operational life, potential electric hazards, and EMI issues.
Leakage current associated with HVDC SSPCs and the “failclosed” nature of SSSD are critical concerns for the safety of SSPC applications. When an SSPC channel is in an “open” state, maintenance personnel may come into contact with an open end of a power channel and get startled due to possible excessive leakage current. This potential safety risk may arise during maintenance activities such as replacing an aircraft load that is connected to the SSPC channel with the excessive leakage. Additionally, an inherent “failclosed” nature of the SSSD can be a safety concern. This concern has become a critical element for the certification of the SSPC technology for commercial aircraft and thus, a secondary means of protection is usually required.
As can be seen, there is a need for an aircraft EPDS which employs SSPCs for control and fault protection wherein vulnerabilities of the SSPCs are mitigated. In particular there is a need for an SPPC-based EPDS with minimal voltage drops, minimal risk of safety issues from leakage currents, minimal risk of nuisance trips from lightning strikes and minimal risk of damage from high inrush currents.